I wanted to control an Arduino with a Raspberry Pi and Node.js using I2C.

WildThumper-I2C

#include <Servo.h>
#include "IOpins.h"
#include "Constants.h"
#include <Wire.h>

//-------------------------------------------------------------- define global variables --------------------------------------------

unsigned int Volts;
unsigned int LeftAmps;
unsigned int RightAmps;
unsigned long chargeTimer=millis();
unsigned long leftoverload;
unsigned long rightoverload;
int highVolts;
int startVolts;
int Leftspeed=0;
int Rightspeed=0;
int Steer;
byte Charged=1;                                               // 0=Flat battery  1=Charged battery
int Leftmode=0;                                               // 0=reverse, 1=brake, 2=forward
int Rightmode=0;                                              // 0=reverse, 1=brake, 2=forward
byte Leftmodechange=0;                                        // Left input must be 1500 before brake or reverse can occur
byte Rightmodechange=0;                                       // Right input must be 1500 before brake or reverse can occur
int LeftPWM=128;                                                  // PWM value for left  motor speed / brake
int RightPWM=128;                                                 // PWM value for right motor speed / brake
int data=1100;
int servo[7];

//-------------------------------------------------------------- define servos ------------------------------------------------------


Servo Servo0;                                                 // define servos
Servo Servo1;                                                 // define servos
Servo Servo2;                                                 // define servos
Servo Servo3;                                                 // define servos
Servo Servo4;                                                 // define servos
Servo Servo5;                                                 // define servos
Servo Servo6;                                                 // define servos


//Reid Setup
int Speed = 200;
int TurnDenominator = 2;
int MoveTime = 1000;
//Serial
String lastMessage = String("");
String inboundMessage = String("");
char newChar;
boolean isMessage;
long lastHello = 0;
int moveInterval = 1000;
String myId = "T1";

int x = 0;

void setup()
{
  //------------------------------------------------------------ Initialize Servos ----------------------------------------------------



  //------------------------------------------------------------ Initialize I/O pins --------------------------------------------------

  pinMode (Charger,OUTPUT);                                   // change Charger pin to output
  digitalWrite (Charger,0);                                   // disable current regulator to charge battery
  
  //i2c for a4 a5
  digitalWrite(18, 1); 
  digitalWrite(19, 1); 

  Speed = 200;

  Wire.begin(4);
  Wire.onReceive(receiveEvent);
  Serial.begin(9600, SERIAL_8N1);
}


void loop()
{
  //------------------------------------------------------------ Check battery voltage and current draw of motors ---------------------

  Volts=analogRead(Battery);                                  // read the battery voltage
  LeftAmps=analogRead(LmotorC);                               // read left motor current draw
  RightAmps=analogRead(RmotorC);                              // read right motor current draw

  if (millis() - lastHello > 20000) {
    handleDiagnostic("hello " + myId);
    lastHello = millis();
  }
  
//  handleInboundSerial();
//  handleLastMessage();
  if (x != 0) {
    handleMove();
    x = 0;
  }
  
} 

//void handleLastMessage() {

void receiveEvent(int howMany)
{
  x = Wire.read();   
  // receive byte as an integer
   Serial.println("integer!");
  Serial.println(x);  
  // print the integer
}  
  
  void handleMove() {
     switch(x) {
      case 1:
        fullstop();
       break;
      case 2:
        goForward();
       break;
      case 3:
        goBackward();
       break;
      case 4:
        goLeftForward();
       break;
      case 5:
        goRightForward();
       break;
      case 6:
        goLeftBackward();
       break;
      case 7:
        goRightBackward();
       break;
      case 8:
        fullstop();
        Speed = 100;
       break;
      case 9:
        fullstop();
        Speed = 200;
       break; 
     }
    
  }
void receiveEventXX(int howMany) {  
    Serial.println("receiveEvent");
      int x = Wire.read(); 
      Serial.println(x);

       }



void fullstop() {
    handleMove(0,0,0,0);
}

void goForward() {
    handleMove(0, Speed, 0, Speed);
}

void goBackward() {
    handleMove(Speed, 0, Speed, 0);
}

void goLeftForward() {
    handleMove(0, Speed, 0, Speed / TurnDenominator);
}

void goRightForward() {
    handleMove(0,Speed/TurnDenominator, 0, Speed);
}

void goLeftBackward() {
    handleMove(Speed, 0, Speed / TurnDenominator, 0);
}

void goRightBackward() {
    handleMove(Speed/TurnDenominator, 0, Speed, 0);
}

void handleMove(int LmotorASpeed, int LmotorBSpeed, int RmotorASpeed, int RmotorBSpeed) {
    //handleDiagnostic("Move LA " + String(LmotorASpeed) + " LB " + String(LmotorBSpeed) + " RA "  + String(RmotorASpeed) + " RB " + String(LmotorASpeed));
    analogWrite(LmotorA,LmotorASpeed);
    analogWrite(LmotorB,LmotorBSpeed);
    analogWrite(RmotorA,RmotorASpeed);
    analogWrite(RmotorB,RmotorBSpeed); 
    delay(MoveTime);
    analogWrite(LmotorA,0);
    analogWrite(LmotorB,0);
    analogWrite(RmotorA,0);
    analogWrite(RmotorB,0); 
    
}

void handleDiagnostic(String message) {
   handleSerialSend("debug", message); 
}

void handleSerialSend(String message, String detail) {
      String toSend = "{ \"addr\" :\"" + myId + "\", \"msg\": \"" + message + "\", \"detail\": \"" + detail + "\" }";
      Serial.flush();   
      Serial.println(toSend);
      Serial.flush();
}